Subsea oil and gas production is growing in importance and is expected to increase significantly in the next 5 to 10 years. In addition, offshore fields are being exploited in deeper and deeper water depths. Floating Production, Storage and Offloading (FPSO) systems are sometimes used to collect the oil and/or gas produced by one or more wells or platforms in an offshore field, process it and store it until it can be offloaded into a tanker or pipeline for transport to land-based facilities. One common approach to FPSOs is to use a decommissioned oil tanker which has been stripped down and re-equipped with facilities to be connected to a mooring buoy and to process and store oil delivered from the wells or platforms. The oil and/or gas is delivered from the well or platform to the FPSO by means of risers, flowlines or export lines connected through a mooring buoy.
Oil and gas production using a FPSO presents many challenges which increase as the water depth increases. For instance, one problem is that the lines used to transfer the oil or gas from a wellhead situated on the seabed to the FPSO are subject to tidal and water current movements and to motions associated with the effects of sea conditions on the FPSO, and therefore can suffer from fatigue or damaging vibrations. Another problem is that the temperature of the oil or gas in the line can change as flow conditions in the line change. As a result at low temperatures, waxes or hydrates can be deposited on the inside of the lines. This is a serious problem especially when, oil or gas production is stopped during shut-in periods. Then the temperature of the oil or gas in the line will cool as a result of heat loss to the surrounding much cooler sea water. In order to prevent hydrates from forming in the lines, some operators have been heating the lines during shut-in periods which are rather costly. Others have been keeping shut-in times too short making maintenance inefficient.
Previous attempts to address these issues have involved modelling of the expected flow-line behaviour and using the results of the modelling to determine insulation and/or heating requirements of the line or maintenance schedules to minimize structural issues. However, these models make many assumptions about the environmental conditions and the pressure and temperature cycles, and in order to reduce the probability of system failure, conservative values or value ranges are applied. This results in costly inefficiencies, overly conservative behaviour and higher running costs. For example, flowlines are often insulated and/or heated to higher temperatures than are necessary which results in additional running costs. Furthermore, shut-in periods are often reduced in time, making it difficult to achieve critical maintenance in one shut-in.
Optical interrogation of fibres is a technology that has been available for many years and there are several commercial applications. In particular, Distributed Temperature Sensing (DTS) which makes use of the Raman backscattered Stokes and anti-Stokes wavelengths (see Brown, G. A. “Monitoring Multi-layered Reservoir Pressures and GOR Changes Over Time Using Permanently Installed Distributed Temperature Measurements”, SPE 101886, September 2006) can provide a distributed temperature measurement along the fibre. This has been used in fire detection applications, power line monitoring and downhole applications. It has also been used on a flexible riser on the subsea platforms or flexible risers connected to an FPSO. Other known techniques for optical interrogation of fibres are the Brillouin and coherent Rayleigh noise (CRN) measurements.
The present invention provides an improved method and system for monitoring the behaviour of subsea lines, such as risers or pipelines. The invention employs distributed measurements with modelling to provide continuous and distributed prediction of subsea line behaviour.